I am starting to see the same on our ooma and on my mother in law's as well since yesterday.
When I all from her ooma to us it goes straight to voicemail. If my sister (who also has ooma) calls us it also goes straight to voicemail. And the same with my mother in law's unit (different location, internet, etc). when we call it goes to vm as well.
No, the do not disturb is not set on any of the units and I also made sure no privacy settings are set.
This started recently.

It happened to me too. Call customer service and ask them to verify the phone number you selected is a valid number. In my case, they determined that the number was invalid and replace it with a new and valid one. The techinician I talked to explained to me that the initial list of phone numbers they offered us during setup is not regularly updated. Unfortunately, I selected the invalid number out of the hundreds in the list. Problem was fixed and I can now receive incoming calls.

If you are set up as modem -> router -> ooma, the issue could be blocked ports. Try connecting the Ooma device directly to the modem or use the port information in this article to to set up port-forwarding in your router.

For modem-> ooma -> router setups, the issue is more likely to be on Ooma's end if you've verified that Do Not Disturb is turned off and already tried rebooting both the modem and the Ooma device several times.

Ooma customer since November 2009.
Formerly employed at another VoIP company.
My opinions are my own.